The present invention relates to a composite material and a method for manufacturing the composite material. More specifically, the present invention pertains to a composite material that is suitable for a heat dissipating substrate on which electronic components, such as semiconductor devices, are mounted, and a method for manufacturing the composite material.
Since electronic components such as semiconductor devices produce heat during operation, such components need to be cooled so that the performance will not lowered. Therefore, semiconductor devices are typically mounted on a base member with a heat radiator plate (heat dissipating substrate) in between.
FIG. 9 shows an aluminum base 41, which constitute a casing, and a heat sink 42, which is secured to the aluminum base 41 by screws (not shown) or by soldering. An insulated substrate 43 is secured to the heat sink 42 by soldering. The insulated substrate 43 has metal (Al) layers 43a on both sides. An electronic component 44 such as a semiconductor device is implemented onto the upper metal layer 43a of the insulated substrate 43 by soldering. The insulated substrate 43 is made of aluminum nitride (AlN). The heat sink 42 is made of a material having a low expansion coefficient and a high thermal conductivity. Specifically, the heat sink 42 is made of metal matrix composite, which has ceramics dispersed in a metal matrix layer. For example, a composite having SiC particles dispersed in an aluminum base material is used.
The metal matrix composite material used for the heat sink 42 is expensive and has low workability. Therefore, a different material for heat dissipating substrates that is inexpensive and has high workability has been proposed. For example, Japanese Laid-Open Patent Publication No. 6-77365 discloses a material for heat dissipating substrates, which is formed by integrating metal plates and a wire fabric sheet. The metal plates are made of Cu, Cu and W (tungsten), or Cu and Mo (molybdenum). The wire fabric sheet is woven with thin metal wires made of Mo or W. FIG. 10(a) shows an example of the material for heat dissipating substrates according to the publication. In this example, metal plates 46 are laid on one another with a wire fabric sheet 45 arranged in between. In this state, the metal plates 46 and the wire fabric sheet 45 are heated and rolled. This integrates the metal plates 46 with the wire fabric sheet 45 and forms a laminated plate 47.
Japanese Laid-Open Patent Publication No. 6-334074 discloses a substrate for semiconductor devices, which substrate includes a base member, in which holes are formed. The base member is made of metal or alloy, the thermal expansion coefficient of which is less than or equal to 8×10−6/° C. The holes are filled with highly thermal conductive material such as metal or alloy, the thermal conductivity of which is more than or equal to 210 W/(m·K). The highly thermal conductive material may be Cu, Al, Ag, Au or an alloy that is chiefly composed of Cu, Al, Ag, or Au. The base member may be an invar plate, which contains 30 to 50% Ni by weight and Fe making up the remaining proportion, or a super invar plate, which contains Co. The holes of the base member are formed by punching after processing the raw material into a flat shape. Alternatively, the holes are formed during casting by the precision casting (lost-wax process).
However, when the laminated plate 47 shown in FIG. 10(a) is extended by applying pressure, spaces Δ are easily formed at portions where the thin metal wires 45a overlap with each other and in the vicinity of the overlapped portions as shown in FIG. 10(b). Air in the spaces Δ deteriorates the thermal conductivity. Also, cracks are easily formed in the wire fabric sheet 45 at the spaces Δ by the repeated thermal expansion and thermal contraction. This reduces the strength of the laminated plate 47. To improve the strength of the wire fabric sheet 45, the contact points of the thin metal wires 45a may be welded. However, it is difficult to weld the contact points of the wire fabric sheet 45, since the wire fabric sheet 45 is woven with the thin metal wires 45a and has fine meshes.
The volumetric ratio of metal having a low thermal expansion coefficient needs to be maximized to suppress the thermal expansion coefficient of the material for heat dissipating substrates. However, in a material using the wire fabric sheet 45, metal exists not only in the meshes, which correspond to holes, but also in portions 47a (see FIG. 10(a)) that correspond to bent portions of the thin metal wires 45a of the fabric sheet 45. Therefore, compared to a structure where a flat metal plate having holes is surrounded with metal, it is difficult to increase the volumetric ratio of a metal having a low thermal expansion coefficient.
The substrate for semiconductor devices disclosed in Japanese Laid-Open Patent Publication No. 6-334074 does not have the drawbacks caused when the wire fabric sheet 45 is used. If holes are formed by punching after processing a raw material into a flat plate, the yield rate decreases, which increases the material cost. Also, forming holes by precision casting (lost wax) increases the manufacturing cost.